I would suggest a good basic seed mix, such as Trill. some eggfood, and Millet..
introduce them to grated fruit, and veg, as they may find the small bits easier to eat, mix it in with a soft mix of egg food, and make it very crumbly. and soft leaves, such as lettuce, or celery leaves.
Dont worry about buying a elaborate seed mix, with anything added, as babies are best on the basic seed mix to start.
Im sure other people feed other things to thier baby budgies, and it will be interesting to hear.